The present invention relates to a cathode ray tube, and particularly to an electron gun for emitting an electron beam, which is mounted in the neck of a funnel.
Generally, a cathode ray tube illustrated in FIG. 1 is formed such that a panel 3 having a shadow mask frame assembly 2 mounted in the inside thereof is joined together with a funnel 4 holding an electron gun 5 in a neck 4a at the end of the funnel. An electron beam emitted from electron gun 5 passes through a deflection region and the aperture of a shadow mask 2a, and then lands on the screen, so that a pixel is formed, and an image is formed by the accumulation of such pixels. A smaller beam spot diameter formed by the above process is favorable to improve the resolution of the image. To further enhance the image's resolution while decreasing the size of the beam spot, the influence of spherical aberration on the electron beam caused by an electron lens should be minimized. If spherical aberration greatly affects the electron beam, a spot halo of a considerable size is formed around the beam spot, thereby adversely affecting the resolution.
FIG. 2 illustrates an example of a conventional in-line type electron gun with a single electrostatic lens. The electron gun 10 has a triode consisting of a cathode 11, a control electrode 12, and a screen electrode 13 which are for generating an electron beam, and a main lens consisting of a focus electrode 14 and an anode 15, for focusing and accelerating the electron beam generated from the triode. Three electron beam passing holes are formed in each electrode of the electron gun, and the central axes of all the electrodes' electron beam passing holes are positioned on substantially the same plane. The diameters of electron beam passing holes 12H and 13H formed in control electrode 12 and screen electrode 13, respectively, are smaller than those of electron beam passing holes 14H and 15H formed in focus electrode 14 and anode 15 constituting the main lens. Generally, the diameter of electron beam passing hole 12H formed in control electrode 12 is smaller than that of electron beam passing hole 13H formed in screen electrode 13.
In the conventional electron gun 10 for the conventional cathode ray tube constructed as above, electron lenses are formed between respective electrodes when a certain voltage is applied to each electrode. A cathode lens C is formed in control electrode 12 supplied with a relatively low voltage potential. A prefocusing lens P for the initial focusing and accelerating of the electron beam is formed between screen electrode 13 of a lower potential and focus electrode 14 supplied with a relatively high potential. Main lens M for the final focusing and accelerating of the electron beam is formed between focus electrode 14 and anode 15 supplied with the highest voltage potential. Accordingly, thermoelectrons are emitted from cathode 11, and the emitted thermoelectrons B are transformed into an electron beam while passing through cathode lens C. The electron beam is preliminarily focused and accelerated while passing through prefocusing lens and is finally focused and accelerated while passing through main lens M, and proceeds to screen 30.
In the conventional electron gun 10 having the above-described structure, since electron beam passing hole 12H of control electrode 12 is smaller than electron beam passing hole 13H of screen electrode 13, electron beam B emitted from cathode 11 passes through the whole region of cathode lens C formed by an electron beam passing hole of a smaller diameter. Therefore, the electron beam formed by having passed through cathode lens is under the influence of spherical aberration from the cathode lens, thereby being incident to main lens M with a large diverging angle. Then, the electron beam incident to the main lens passes through the whole region of the main lens, from its center to the periphery, so that the incident electron beam comes under great spherical aberration influence. As a result, when the electron beam greatly affected by spherical aberration reaches the screen, a beam spot having a halo somewhat darker than the center and of considerable size, forms around its bright core.